Colloidal fillers have been widely used in industry for almost a half a century as reinforcing agents for rubber and other polymers. Fillers such as silica and carbon provide useful reinforcing agents for rubber due to their colloidal structure. Colloidal structure is primarily a function of the colloidal interactions which define the morphology and strength of the colloidal structure. Colloidal interactions are interactive chemical and physical forces between individual particles, and between clusters of particles. Such chemical and physical forces typically include chemical bonds, and ionic associations such as hydrogen bonding, hydrophobic interactions, electrostatic interactions and Van der Waal's forces. The interrelationship of these forces ultimately defines the overall structural characteristics of the filler.
Silica, as compared with carbon black, the traditional reinforcing agent, offers not only improved mechanical properties as a reinforcing filler in polymer applications, but also the possibility of the polymer being translucent or neutral in color. Silica, therefore, has been the preferred filler for reinforcement in many rubber and polymeric materials. Commercially available silica is typically present as precipitated silica. Precipitated silica has a three-level structure, composed of primary particles, aggregates and agglomerates. Primary particles are the first to form during synthesis of silica and the smallest structural component, typically less than one micron in size. Primary particles tend to cluster together during synthesis to form aggregates which are also typically less than one micron in size. Generally, aggregates form from intermolecular chemical bonding between individual primary particles. Chemical bonds are robust making the aggregates “hard”, in the sense that they do not easily break up under shear and pressure. Primary particles and aggregates play a crucial role in polymer reinforcement.
Silica precipitate also exists in an agglomerated state. Agglomerates are 100 μm clusters of aggregates. Generally, agglomerates form from physical bonding between aggregates during the synthetic process. Physical bonds are not as robust as chemical bonds and are easily broken down during shear. Agglomerates tend to be much larger and “softer” than the corresponding aggregates. Physically bound agglomerates in and of themselves are fairly ineffective as reinforcing fillers due to their softness and large size. However, formation of agglomerates provides convenience for packaging and transportation of the final silica product.
During the rubber reinforcement process, however, the colloidal structure of the precipitated silica filler is exposed to tremendous shear and disruptive forces. Shear causes “soft” agglomerates to be broken down into corresponding “hard” aggregates. This leads to improved dispersion of the silica reinforcing filler. Therefore, to improve the performance of a reinforced rubber, an ideal silica filler would be composed of both “hard” aggregates, to withstand the pressures of shear during rubber manufacture, and “soft” agglomerates, to allow for their breakdown during dispersion in the rubber matrix.
The synthesis and formation of the colloidal structure of precipitated silica may be systematically controlled using the tools of chemistry. While chemically bound aggregates are favorable, the size of the primary particles as well as the formation of aggregates and corresponding agglomerates are very important for dispersion of precipitated silica in rubber matrices. The prior art has also attempted to study the effects of varying reaction conditions on the structure of silica in order to obtain the desired structural composition and characteristics of primary particles, aggregates and agglomerates. References Chevallier U.S. Pat. No. 5,403,570, and Groel U.S. Pat. No. 5,705,137 disclose precipitated silica of varying colloidal structure as potential reinforcing agents. Chevallier discloses modified silica produced in an effort to improve dispersive properties. Groel discloses precipitated silica preparing from synthetic methods in which the pH and temperature conditions were varied. These references, however, fail to improve the colloidal structure of silica as related to reinforcement, and in particular, fail to effectively control the formation of aggregates and agglomerates of the silica precipitate.